Land management influences the effects of fire on soil properties: implications for post-fire restoration efforts

Climate change, at the rate at which it is occurring, is having devastating impacts around the globe. In Australia, climate change has led to rising fire frequency, fire severity, area burned, and area susceptible to burning. These changes are causing a net loss of soil functionality across the country, thus threatening Australia’s agricultural productivity, ecosystem biodiversity, resiliency to climate disasters, clean air and water, and copious other ecosystem services. As fire regimes in Australia continue to change, it is becoming more important to understand the impact of land use on post-fire outcomes. As of 2010, approximately 84% of Australian land was being managed, e.g., more than 40% had been cleared and over 50% was used for grazing.  On its own, fire can cause loss of vegetation and dependent ecosystem services, such as food and habitat, evapotranspiration and climate stabilization, and carbon sequestration, increased hydrophobicity, altered microbial communities, and soil erosion. Land clearing also results in loss of vegetation, and can lead to soil erosion, nutrient run-off, and threatened water quality. Additionally, grazing practices can increase soil nitrogen, promoting weed growth and soil acidification, and cause soil compaction, hindering native vegetation, increasing water run-off, and promoting soil erosion. However, very little is known regarding how soil is affected when both land management and fire act together.

Between December 16^th, 2019 and January 30^th, 2020, 46% of Kangaroo Island, a Mediterranean-climate region off the coast of South Australia, was burned by a megafire. This megafire affected both managed, e.g., cleared and grazed, and non-managed land across the island including multiple areas of our study site, located at 35°43’S, 137°00’E. The objective of this study was to better understand the interactive effects of land management and fire on the soil functionality of these Kangaroo Island sites to help land managers restore the burnt and grazed grasslands to a native vegetative state. Within our study area, 14 sites were identified: four burnt, cleared, and grazed grassland sites, five burnt and non-managed sites dominated by Eucalyptus, and five unburnt and non-managed sites dominated by Eucalyptus. Six months after the Kangaroo Island megafire, replicate soil samples (n=10) from the top 5cm were collected from each of the 14 sites. Samples were transported to the laboratories at The University of New South Wales for physicochemical and microbiological analysis, e.g., pH, electrical conductivity, hydrophobicity, aggregate stability, total nutrient content, and microbial abundance, community composition and diversity. Our preliminary results showed significantly higher hydrophobicity (p < 0.01), as well as total carbon, total nitrogen, and microbial activity (p < 0.001), and significantly lower pH (p < 0.01) in soils collected from burnt, cleared, and grazed plots compared to burnt non-managed plots. These results suggest that pre-fire land management has a significant influence on how fire affects soil health, providing valuable insights that will guide the restoration effort of our study area and serve as an example for others.